This invention relates to a heating device. More particularly, the invention relates to a self regulating heating device. In still greater particularity, the invention relates to a self regulating heater using a positive temperature coefficient (PTC) resistive material specifically adapted for use in heating automotive-type outside rearview mirrors.
Heating devices for glass plates including mirrors using positive temperature coefficient materials have been devised. Two such devices are disclosed in U.S. Pat. Nos. 4,628,187 and 4,631,391. These devices have certain disadvantages and shortcomings which the present invention overcomes. For example the device in U.S. Pat. No. 4.631,391 uses individual spaced apart platelettes of PTC heater elements sandwiched between two heat conductive layers which do not provide uniform heating of the surface to be heated. In the case of U.S. Pat. No. 4,628,187, an area principally at the periphery of the mirror occupied by the electrode material of the heating device is not heated resulting in a significant reduction in mirror heated area. Further, it should be noted that the electrode system in this device uses substantially wide. Constant width silver buss bar conductor paths to carry the necessary current between the terminal connections and the electrode system. The wide conductors not only result in significant "cold" areas of the mirror along the length of the conductors, but also requires significant quantities of the precious metal silver which significantly adds to the cost of the device.